Method for the biodegradation of organic contaminants in a mass of particulate solids

ABSTRACT

A method for the biodegradation of organic contaminants in a mass of particulate solids. The method comprises providing a contaminated mass of particulate solids on an impervious surface in fluid communication with an impervious recovery reservoir. The impervious surface has operating thereon air supply and/or air suction means to provide suitable and continuous oxygenation of the mass and/or to remove undesirable vapor emissions from the mass. The mass is periodically irrigated by applying on its surface a culture medium comprising at least one bacterial strain and co-substrates thereof. The bacterial strain has the ability to degrade the organic contaminants. The medium is drained through the mass and washings recovered in the reservoir to be reused to irrigate the mass. The mass is periodically mixed and its temperature and moisture level are controlled to monitor the biological activity of the bacterial strain and to maintain the activity at levels suitable for the bacterial strain to degrade the contaminants in the mass.

This application is a continuation of application Ser. No. 07/710,206,filed Jun. 5, 1991, abandoned.

FIELD OF THE INVENTION

The invention relates broadly to the degradation of organic contaminantspresent in particulate solids. More particularly, the invention relatesto the degradation of organic contaminants in soils throughmicrobiological processes.

BACKGROUND OF THE INVENTION

Bioremediation is the use of living organisms for detoxication ofhazardous wastes. It can involve either the introduction of specificorganisms and/or the stimulation of indigenous bacteria. The techniquewas first experimented in the 1970's to treat contaminated soils andaquifers. However, hurdles in the regulatory processes and constantpressure from environmentalist groups had reduced the use ofbioremediation to marginal levels.

In recent years however, several advantages have been recognized forbioremediation, particularly in situ bioremediation. Among others, ithas been found that the technology is specific, relatively cheap andquicker to use than most other remediation techniques. The technique canbe used to degrade a wide variety of organic compounds present atvarious levels in contaminated soils. One of the most importantadvantages of biological degradation over conventional techniques is thefact that the contaminants are usually broken down to harmlesssubstances whereas conventional techniques usually only temporarydisplace the problem or transfer the contaminants to another medium.

Various approaches to bioremediation of soils have been tested andcompared. One preliminary conclusion that seems to be drawn from thosetests is that microorganisms occurring in nature are so versatile and soadaptable that most applications of bioremediation could rely almostexclusively on the use of natural organisms that have not been modifiedin any way, thereby avoiding, at least for now, the introduction innatural systems of genetically engineered organisms.

The most common technique used in bioremediation involves thestimulation of naturally occurring bacteria residing at the site ofcontamination. It is usually called "biostimulation". By addition of theappropriate nutrients, principally oxygen, phosphorus and nitrogen, andby maintaining optimum growth conditions, it is possible in someinstances to favour increased multiplication of the communities ofindigenous organisms that together are capable of degrading undesirablecontaminants. Hence, the addition of microorganisms to the site is notrequired in biostimulation. A common variation of the "biostimulation"technique is land-farming. In land-farming, chemical nutrients are addedto soil, often in an excavated pile, while adequate oxygenation isassured by frequent turning or dishing of the soil.

Implementing biostimulation usually requires a certain amount oflaboratory testing, to ensure that there are sufficient numbers ofindigenous microbes on site, and to determine the optimum conditions toenhance their growth and biodegradative action. This preliminarycharacterization usually leads to treatability studies to establish thatthe site can be remediated economically. In situ bioremediation usuallyrequires extensive engineering to introduce nutrients to the site,through injection wells, infiltration galleries and the like.Land-farming, in contrast, can be accomplished simply by spraying thenutrients onto soil piles that are frequently mixed and aerated.

In U.S. Pat. No. 4,849,360, Norris et al. describe a process usingbiostimulation for confining contaminated soils and degrading thehydrocarbons they contain. The method comprises the use of indigenousmicroorganisms, nutrients, water and a suitable gas distribution system.The amount of contaminated soils to be treated is adjusted by evaluatingthe capacity of the gas distribution system to create optimal aerobicconditions (see column 2, lines 47-50). The method also involvesevaluating the native microbial community in the soil and creatingproper conditions for this community to grow as much as possible (seecolumn 4, lines 12-45). One of the major drawbacks of this methodappears to be the fact that the soils to be decontaminated must beconfined in a container.

Another example of a decontamination system using biostimulation isdescribed in the March 1991 issue of Chemical Engineering in an articleentitled "Mighty Microbes". For this system, liquids are sprayed on thecontaminated soil pile, and air may be blown or suctioned through themass by a system of pipes located under the mass. This is described as a"wet" technique for the onsite processing of soil. Contrary to thetechnique described in U.S. Pat. No. 4,849,360, soil is not confined toa container. The sprinkling system is used to add water and nutrientsand the air distribution system buried in the pile increases oxygensupply. Although the method has proven to be quite interesting for thedegradation of some contaminants, it has been found to be relativelytime consuming and somewhat limited for degrading more chemically stablecontaminants such as PCB's. As shown at page 33 of the article,decontamination of a contaminated soil containing PCP and creosote ledto a 58% reduction in contaminants in 3 months. Generally, this is notsufficient to meet the levels required by the regulatory authorities inNorth America.

An alternative to biostimulation is "bioaugmentation". This techniqueconsists in introducing non-native cultures, previously selected fromother sites for their ability to degrade specific wastes. Thesemicrobial products are usually blends of different species or strains.In recent years, companies have begun selling microbial blends purportedto be active against hazardous compounds, including use for in situwaste remediation. Most common are products for degradation ofhydrocarbons and petroleum distillates, but several manufacturers alsosell microbial products with claimed activity against aromatic compoundsand other hazardous chemicals. In addition to these commerciallyavailable cultures, there are several microbial isolates that have shownsuccess in the laboratory in degrading hazardous wastes, such as thewhite-rot fungus, which can degrade lignins and many other aromaticcompounds.

Waste treatment technologies based on the principle of bioaugmentationhave also been developed. In U.S. Pat. No. 4,850,745, Hater et al.describe a bioaugmentation technique by which a system for treating soilcontaminated by petroleum hydrocarbons is designed by applying in a dryform a suitable bacterial culture capable of degrading petroleumhydrocarbons to the bottom of an excavated cavity. A system ofdistribution piping capable of supplying nutrients directly to thecultures and also an air flow through the area containing the culturesis provided to maintain optimal growth conditions. The system describedby Hater et al. seems to be operated in a closed circuit. In otherwords, Hater et al. do not teach or suggest the subsequent introductionof microorganisms once the initial inoculation has been made.

In U.S. Pat. No. 4,952,315, another type of bioaugmentation technique isdescribed. Saab discloses a process for eliminating hydrocarbonscontained in a contaminated soil. The desired result is obtained byusing a microbiological treatment involving the use of emulsifierspermitting the separation of the contaminants from the soil in whichthey are found. The contaminants can then be degraded by using abiological process requiring endogenous bacteria. This approach can besomewhat lengthy as it is required to bring the contaminants in a fluidphase before having the possibility of degrading them through the actionof microorganisms.

One of the most promising applications for bioaugmentation appears to bein the degradation of oil spills, since the biology of hydrocarbondegradation has been well studied. Unfortunately, most of the methodsthat were used so far to decontaminate major oil spills such as the MegaBorg and Exxon Valdez spills could not generate conclusive data.Furthermore, although bioaugmentation allows the introduction ofmicrobes tailored for a given waste, it has difficulty working inpractice because competition from natural microbial populations requireslarge inoculum sizes, and because cultured organisms cannot alwayshandle the stresses present in natural environments.

Bioremediation offers some concrete advantages over competing methods.It is a destructive technology that offers a permanent solution tohazardous waste problems, without the need to remove the wastesoff-site. It utilizes a natural process that does not itself createenvironmental problems. Even though in situ biostimulation requirespreliminary laboratory assessment, it can be implemented quickly andinexpensively at most sites. Soil bioremediation has been estimated tobe far less expensive than incineration or land disposal, and competeswell with other available options, like recycling.

However, bioremediation, either through biostimulation orbioaugmentation, has its limitations. Hence, bioremediating soil willgenerally take longer than excavation for incineration or landfilling.Also, biostimulation is often insufficient to provide degradation ofcontaminants at acceptable levels while in the case of bioaugmentation,one of the major problems seems to reside in the fact that it isdifficult to maintain the cultures introduced at the beginning of theprocess to optimal levels.

SUMMARY OF THE INVENTION

In accordance with the present invention, it has been found that theperiodic irrigation of a contaminated mass of particulate solids with aculture medium which may contain one or more appropriate bacterialstrains endogenous or indigenous to the contaminated mass and having theability to degrade organic contaminants leads to substantialimprovements in yield when compared to biostimulation techniques andconventional bioaugmentation techniques in which the mass to bedecontaminated is inoculated only once.

The present invention therefore relates to a method for thebiodegradation of organic contaminants in a mass of particulate solids.The method comprises providing a contaminated mass of particulate solidson an impervious surface in fluid communication with an imperviousrecovery reservoir. The impervious surface has operating thereon airsupply and/or air suction means to provide suitable and continuousoxygenation of the mass and/or to remove undesirable vapor emissionsfrom the mass. The contaminated mass is then irrigated by periodicallyapplying on its surface a culture medium which may comprise at least onebacterial strain and cosubstrates thereof. The bacterial strain iseither endogenous or indigenous to the contaminated mass and has theability to degrade the undesirable organic contaminants. The medium isdrained through the mass and recuperated in the recovery reservoir andthe washings are to be reused to irrigate the contaminated mass. Themass is periodically mixed and its temperature and moisture level arecontrolled to monitor the biological activity of the bacterial strainand to permit adjustment of the biological conditions inside the mass tomaintain bacterial levels suitable for the strains to degradeundesirable contaminants. The expression "culture medium" when usedherein is entitled to designate a solution containing various nutrients,co-substrates and optionally surfactants to be added to the contaminatedmass as well as a vehicle for the bacterium inoculates that may beintroduced periodically to the contaminated mass.

Hence, the process of the present invention provides a method by whichlevels of active microorganisms can be maintained in the contaminatedmass to maintain decontamination conditions which are as optimal aspossible. The bacterial culture applied on the contaminated mass throughirrigation can be applied either continuously or at periodic intervals,depending on the overall level of contamination and the desireddecontamination time.

Preferably, the impervious surface used in the method of the presentinvention is sloped to enable recovery of the washings drained throughthe contaminated mass. Also, the air supply and/or air suction systempreferably comprise a series of perforated conduits connected to an aircompressor operable in suction and/or compressed mode. The perforatedconduits are preferably located inside a gravel bed and the compressorhas an activated charcoal filter which is operated when the system is inthe suction mode.

The culture medium applied to the mass may comprise specific nutrientssuch as nitrogen and phosphorus and surfactants to improve theefficiency of the microbial cultures present in the medium and in thesoil. More preferably, the bacterial medium is provided from a mediumdelivery unit allowing to maintain the strains used to decontaminate themass or from the recovery reservoir in which it is introduced prior toinitiating decontamination. The culture medium may be applied on themass by being pumped from the delivery unit or the recovery reservoirthrough a plurality of sprinklers or a perforated double-partitionresilient irrigation tubing network in fluid communication with thedelivery unit. The sprinklers and resilient tubing are respectivelylocated above and on the mass to be decontaminated. The cultureirrigated through the mass can be reused by being pumped from therecovery reservoir through the sprinklers or the tubing network. Therecovery reservoir can also combine the dual function of delivery andrecovery, in which instance the culture medium is directly introduced init and combined with the washings to irrigate the contaminated mass. Thetemperature and moisture level of the mass may be measured by aplurality of thermocouples and appropriate sensors inserted in the mass.Other preferred features include the direct oxygenation of the recoveryreservoir to maintain the bacterial population present in both theculture medium and the washings as well as the use of an imperviouscover placed over the contaminated mass to segregate rain water and toassist in maintaining optimal temperature levels.

Also within the scope of the present invention is a system for thebiodegradation of organic contaminants in a mass of particulate solids.A known system for the biodegradation of organic contaminants comprisesa sloped impervious surface having thereon air supply and/or air suctionmeans to provide suitable and continuous oxygenation of the mass or toremove undesirable vapor emissions from the mass. It also comprises animpervious recovery reservoir in fluid communication with the slopedimpervious surface, a storage container having therein a solutioncontaining nutrients and spraying means connected to the storagecontainer to irrigate the mass by spraying the nutrients on the mass,the solution being drained through the mass and recovered in thereservoir to be reused to irrigate the mass.

In the system of the present invention, one improvement comprisessubstituting the storage container by a medium delivery unit in which aculture medium may comprise at least one bacterial strain andco-substrates thereof is maintained. The system of the present inventionalso comprises means to continuously measure the temperature andmoisture level of the mass to monitor the biological activity of thebacterial strain and to maintain the activity at levels sufficient forthe bacterial strain to degrade the contaminants in the mass. Asmentioned previously, the means to measure the temperature and moistureof the mass may preferably comprise a series of thermocouples insertedin the contaminated mass. Means to provide suitable oxygenation of theimpervious recovery reservoir which, as mentioned previously, may beused alone and combine the dual delivery-recovery function, are alsoprovided.

The following is a description by way of example of preferredembodiments of the present invention, reference being had to thefollowing drawings in which,

FIG. 1 is a side elevation of a preferred embodiment of the biotreatmentsystem of the present invention;

FIGS. 2 and 3 represent a comparison of the temperature inside andoutside the contaminated mass at various intervals during thedecontamination of hydrocarbons using the process of the presentinvention;

FIG. 4 represents a comparison of the temperature inside and outside thecontaminated mass at various intervals during the decontamination ofhydrocarbons using the process of the present invention when thecompressor was operated in the suction mode;

FIG. 5 represents oil and grease concentration throughout thedecontamination of hydrocarbons using the process of the presentinvention;

FIG. 6 represents chromatograms showing the progression of PAH'sdegradation during the decontamination of hydrocarbons using the processof the present invention;

FIGS. 7 and 8 represent side elevations of another embodiment of thebiotreatment system of the present invention; and

FIG. 9 represents a plan view of the biotreatment system of FIGS. 7 and8;

FIG. 10 represents the reduction of PCP concentration during thetreatment of PCP contaminated soil using the process of the presentinvention;

FIG. 11 represents the reduction of oil and grease concentration duringthe treatment of PCP contaminated soil using the process of the presentinvention; and

FIG. 12 represents the reduction of PCP concentration in washings fromthe treatment of PCP contaminated soil using the process of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for the biodegradation oforganic contaminants in a mass of particulate solids through continuousapplication of culture medium on the mass to be decontaminated. It alsorelates to a system for carrying such method. The parameters thatusually affect the efficiency of biodegradation of various wastes suchas oily waste residues include the presence of active organisms, oxygensupply, the addition of nutrients, the use of surfactants, the presenceof co-substrates, the temperature, the moisture level and the pH. Hence,the choice of the site design must take most of these parameters inconsideration to allow maximum efficiency of the bioremediation process.

Referring now to FIG. 1, a preferred embodiment of the decontaminationsystem of the present invention, generally designated by referencenumeral 10, comprises a sloped impervious surface 12. The impervioussurface 12 is preferably made of a material such as concrete, asphalt,polyethylene or other suitable polymeric plastic and has a sufficientinclination to permit recovery of the washings, preferably between 1 and3%. The impervious surface 12 is connected to an impervious recoveryreservoir 14 in which culture medium washings may be collected. On theimpervious surface 12 is installed an air distribution system comprisinga series of perforated pipes 16, 18, 20, 22 and 24 which are placed in agravel bed 26. The gravel bed is optional but it appears to providebetter distribution of the air provided to the contaminated mass. Theperforated pipes 16, 18, 20, 22 and 24 are linked to pipe 28 leading toan air compressor 30. The compressor 30 may be chosen to allow operationin suction mode as well as compressed mode. In both the suction and thecompressed mode, air is provided to maintain suitable aeration of thecontaminated mass. However, the compressed mode may also serve tocontrol the air temperature and hazardous vapor emissions if required.When the compressor is operated in compressed mode, air is passedthrough pipe 32 and when the compressor is operated in the suction mode,air is passed through pipe 34 and activated charcoal filter 36 or a peatabsorbent filter (not shown) to recover gaseous contaminants.

The decontamination system 10 also comprises an irrigation system,generally designated by reference numeral 38. The irrigation system 38comprises a medium delivery unit 40 to provide nutrients, surfactants,bacteria inoculates and required co-substrates to the contaminated mass.The irrigation system 38 also comprises a series of sprinklers 42, 44,46 and 48 in fluid communication with both the delivery unit 40 and therecovery reservoir 14. Pumps 50 and 52 are provided to supply culturemedium to the sprinklers 42, 44, 46 and 48 through either the deliveryunit 40 or washings accumulated in the recovery reservoir 14.

To operate the decontamination system of FIG. 1, a decontamination siteshould first be constructed with a proper slope on which is installedimpervious surface 12 in such a fashion as to allow collection ofwashings in the recovery reservoir 14. Perforated pipes 16, 18, 20, 22and 24 are then placed on the impervious surface 12 and covered with agravel bed 26. The contaminated mass of particulate solids 54 is thenpiled on the gravel bed 26 and irrigated with a culture medium pumpedfrom the delivery unit 40 through pump 50 and sprinklers 42, 44, 46 and48. As the microorganisms contained in the culture medium diffusethrough the contaminated mass 54 in the direction indicated by thearrows 56 and 58, compressor 30 is operated to provide suitable oxygenlevels to maintain optimal activity of the microorganisms introduced inthe soil and to provide sufficient oxygenation for the organisms alreadypresent in the contaminated mass. Once the culture medium pumped throughthe sprinklers from the delivery unit 40 has reached the bottom of thecontaminated mass 54, its washings are drained in the recovery reservoir14 and may be pumped through pump 52 to be reused again. Drained culturemedium (or washings) is therefore recirculated through this system andanalysis of phosphorus and nitrogen levels found in the washings areused to optimize fresh medium addition.

The frequence at which the contaminated mass is irrigated and the amountof culture medium that is provided depends on various factors includingthe level of contamination of the particulate mass and the time withinwhich the contaminated mass is to be decontaminated. Preferably, theculture includes nitrogen, preferably of a concentration between 3 and 5ppm, phosphorous, preferably at a concentration of 0.3-0.5 ppm.

The bacterial strains that are used are selected for their ability todegrade the contaminants present in the mass of particulate solids.Incubation of the bacterial strain can be done on-site and thermocoupleslinked to a digital data collection system as well as suitable sensorsare respectively used to monitor temperature and moisture levels in thecontaminated mass.

Another embodiment of the decontamination system of the invention isshown in FIGS. 7, 8 and 9.

Referring to FIG. 7, the decontamination system, generally designated byreference numeral 70, has an impervious surface 92 which usuallypresents an inclination of 1 to 3% in order to provide suitable recoveryof the culture medium washings. The impervious surface 92 is preferablyeither a polyethylene membrane or a concrete or asphalt lining. Theimpervious character of the sloped surface 92 is particularly importantto avoid the introduction of contaminants in non-contamined soil. Theimpervious surface 92 may be sloped in such a manner as to taper fromthe center of the contaminated mass towards each of its side or from oneside of the contaminated mass to the other. The impervious surface 92leads to a concrete channel 94 in which the washings are collected. Thechannel 94 may contain gravel (not shown). The channel 94 is protectedby cover 88 to prevent rain water to be introduced in the system.Channel 94 is connected to a recovery reservoir (not shown) that is usedto contain the culture medium and to recover the washings from thedecontamination process. The recovery reservoir may be oxygenated inorder to provide adequate conditions for the microorganisms found in thewashings to degrade the contaminants present in the washings. Hence, theculture medium and the washings contained in the recovery reservoir canbe oxygenated either by providing air through suitable air supply means(not shown) or by adding chemical substances such as hydrogen peroxide.The culture medium and the washings are sprayed on the contaminated massby being pumped from the collecting reservoir through the irrigationsystem 82 using a pumping system similar to the one used in the previousembodiment.

A cover 88 is placed over the contaminated mass and the irrigationsystem 82 which is described in further detail later on. Preferably, thecover 88 may be an impervious cloth which may be maintained in positionusing weights such as sand bags 90 that prevent the cover 88 from beingblown loose by winds. Another arrangement that can be used to maintainthe cover 88 in position is to provide a hose (not shown) filled withwater or any suitable liquid, circling the cover 88 at the bottom of thepile. The cover 88 is used for at least three purposes. Firstly, itallows segregation between the rain waters and the waters used in thedecontamination process. Secondly, it prevents the escape of volatilecomponents generated from the decontamination process. Thirdly, itprovides an isolation feature that enhances the overall efficiency ofthe heating of the mass.

The air distribution system, generally designated by reference numeral72, comprises a main pipe disposed longitudinally to the pile anddesignated by numeral 74 (FIG. 9). A series of perforated transversalpipes 78 are connected to the main pipe 74 to provide even oxygenationof the contaminated mass.

The transversal pipes are preferably spaced apart by a distance of about3 meters. Each of the perforated pipes is controlled by suitable valvemeans 100 (FIG. 7). The perforated pipes 78 are located underneath thecontaminated mass and on the impervious surface.

The use of gravel to cover the perforated pipes 78 is foreseeable butoptional, although in some instances it may increase the speed at whichdecontamination is carried out. However, in instances where treated soilis to be removed and new contaminated soil is to be added to the site,the use of gravel does not appear to be practical. This decrease in airdistribution efficiency incurred from not using gravel can be obviatedby increasing the number of perforated transversal pipes.

The perforated pipes 78 are linked to an angled member 76 which islinked to the main pipe 74. The main pipe 74 is linked to an aircompressor (not shown) which is similar to the air compressor describedin the embodiment shown in FIG. 1.

The irrigation system, generally designated by reference numeral 82, ismade of a series of perforated double-partition irrigation tubes 84 and86 which are best illustrated in FIG. 9. Referring to FIG. 9, theirrigation system 82 comprises a series of transversal and longitudinalperforated double-partition irrigation tubes respectively designated byreference numerals 84 and 86. The perforated double-partition irrigationtubes 84 and 86 are usually made of a flexible material similar to thematerial used to make regular watering hoses.

The tubes 84 and 86 are interconnected through a series of valves (notshown) that allow for the partial operation of the irrigation system 82.This type of irrigation system allows for continuous operation of theapparatus. Hence, in instances where the contaminated mass to be treatedis not provided all at once, the system can be partially operated andthe treated mass can be removed and replaced by additional contaminatedmass. This makes the overall decontamination operation more flexible.

Contrary to the embodiment described previously, the irrigation systemis not provided with sprinklers. However, the double-partitionirrigation tubes 84 and 86 allow for a slow release of the culturemedium, which cannot be done easily when sprinklers are used. By usingthese tubes, the resulting irrigation allows for a more permanentcontrol of the moisture level of the contaminated mass as the irrigationrates can be brought down to very low levels. This is not possible whensprinklers are used, as one has to irrigate the system and stop untilthe sprayed culture medium is infiltrated in the contaminated mass. Atthe beginning of the decontamination process, the system is operated inthe suction mode. The irrigation rates, when using the secondembodiment, may be such that continuous irrigation is possible. Theculture medium itself can be pre-oxygenated, thereby eliminating theneed to initially operate the system on a compression mode. Whenvolatile substances are no longer at undesirable levels, the compressionmode may be used. If the system is operated in cold temperatures, thecompressed air is warmer than the ambient temperature and consequentlyheats the contaminated mass. As mentioned previously, the use of thecover 88 also allows better conservation of the heat generated by thecompressed air.

Although it may be useful to constantly irrigate the contaminated masswith the culture medium when the decontamination process is initiated,constant irrigation is not required throughout the entiredecontamination process. Hence, irrigation rates may be controlled intwo preferred manners. Firstly, a timer may be used to activate theirrigation system at regular intervals, for example every 12 hours, fora predetermined period of time, for example 1 hour. The rate ofirrigation and the volume of culture medium to be used is calculated andadjusted depending on the hydraulic conductivity of the soil andaccording to the type of soil to be decontaminated. Alternately, amoisture indicator can be inserted in the contaminated mass and set toactivate the irrigation system when the moisture reaches a certainlevel. The rate at which the culture medium may be provided variesdepending on the amount of liquid which can be reatined by theparticular soil under study. This capacity is at least a function of thesize of the particles contained in the soil. This varies substantiallydepending upon the type of soil. For instance, clays will retain muchmore water than sands. All the poured liquid leaches out after havingsaturated the soil with water and waited 24 hours before pouring if thepouring rate is not superior to the infiltration rate. Hence, themoisture indicated is to be adjusted to operate only when water in thecontaminated mass has an infiltration pouring rate similar to theinfiltration rate. In other words, the rate of pouring of the medium isa function of the infiltration rate of the soil to be decontaminated.The rate at which the culture medium is added is not particularlyrelated to the level of contaminants found in the contaminated soil, thepurpose being to effect decontamination as quickly as possible. Ideally,soil should be constantly irrigated as long as its macropores are filledwith air. This is done by measuring the "field capacity" (capacite auchamps) of the soil to be decontaminated.

The volume of contaminated mass that can be decontaminated at once usingeither embodiment of the system of the present invention variesdepending upon various parameters, at least as far as width and lengthof the mass are concerned. The length of the contaminated mass is notcritical and may be adjusted depending on the size of the system. Withregard to the width, it is important to consider that, as thecontaminated mass has to be turned periodically, the width may vary withthe means available to turn the mass. Generally, preferred width variesbetween 9 and 14 meters. As far as the height of the contaminated massis concerned, the optimal height will range between 2 and 3 meters.Heights exceeding 3 meters can be foreseen but the efficiency of thedecontamination process is usually reduced as irrigation and oxygenationof the contaminated mass becomes more difficult. On the other hand,heights below two meters are not interesting as the volume that can bedecontaminated at once is reduced, thereby increasing the costs of theoverall operation. Furthermore, a contaminated mass of reduced heightexhibits less thermal inertia. Decreases in thermal inertia usually meanthat during cold nights, the overall temperature of the contaminatedmass would fall too rapidly.

The volume of air that may be introduced to oxygenate the contaminatedmass may vary from 1/2 to 2 times the overall volume of the contaminatedmass an hour. Optimally, the volume of air introduced in thecontaminated mass every hour should be the same as the volume of thecontaminated mass itself. The air can be heated to maintain optimaldecontamination conditions. This may be particularly interesting ifdecontamination is to be effected in cold climates.

It is important to mention that the compressor may also be operatedpermanently or controlled using a suitable timing system. For example,when treatment of the contaminated soil is almost completed, not as manyair changes are required as not as many contaminants are to be degraded.As mentioned previously, at the beginning of the decontaminationprocess, oxygen can be provided by adding an oxygenated culture mediumin order to limit volatilization of the gaseous contaminants. Anothermanner in which the escape of volatile gases may be controlled is byintroducing a sensor in the contaminated mass to determine the rate ofvolatile products. When the rate of escape of volatile contaminants isat high levels, oxygenation through the compressor may be interruptedand restarted when the rate has decreased to acceptable levels.Typically, the level of volatile contaminants should be maintained below260 mg/m³ an hour for benzene, 300 mg/m³ an hour for naphtalene and 200mg/m³ an hour for toluene for example.

With regard to the microorganisms used in the decontamination process,various types of organisms may be used depending on the type ofcontaminant present in the contaminated mass. Generally speaking, thestrains used are isolated from contaminated sites and assayed for theirability to degrade a given contaminant. They may be used either as purecultures or as mixed cultures, being mixtures of microorganisms. In thecase of hydrocarbons, for example, various Pseudomonas strains such as,Pseudomonas putida may be used. In the case of PCPs, various bacterialstrains can be used as well as various spores such as Coriolusversicolor and Sphanerochaete chrysosporium. Although the selection ofthe strain is of some importance, the method by which a given strain isselected to be used in bioremediation is relatively well known by thoseskilled in the art.

The pH of the contaminated soil usually has to be maintained between 6and 8. Many ways can be used to maintain the required pH that will notbe harmful to the organisms in the culture. One example is to use lime.

Anionic or non-ionic surfactants can also be added to the contaminatedsoil in preferred concentrations of about 0.01% in the culture medium inorder to reduce surface tension. The types of surfactants that may beused are commercially available surfactants that are well know by thoseskilled in the art.

The growth medium is incubated after inoculation with the proper culturefor a sufficient period of time to allow the microorganisms to grow. Themicroorganisms may be cultured in a laboratory to a high concentrationto form a stock solution. Alternately, the microorganisms may becultured only until a suitable microorganism culture suspension forcarrying out the process is achieved. For example, in the laboratory,microorganism concentrations of about 1×10¹⁰ can be produced and dilutedto 1×10⁶ or 1×10⁷ prior to being used to irrigate the contaminated mass.The culture medium can be maintained on the site by using appropriateco-substrates.

As far as inoculation rates are concerned, it has been found thatregular inoculation provides more adequate conditions for achievingsuitable decontamination levels. Whereas most prior art processesprovide designs that allow only a single inoculation of the contaminatedsoils, the process and system of the present invention contemplatesperiodic inoculation. Preferably, inoculation can be effected every twoweeks as an average and typically, the concentration of microorganismsintroduced in the contaminated mass ranges from 10⁶ to 10⁷ per cubicmeter of water for each 100 m³ of contaminated mass.

Important aspects that have been developed to improve bioremediation oforganic contaminants include the use of a fabric cover, the possibilityof constantly irrigating the contaminated mass by providing an apparatusallowing low irrigation levels, the possibility of oxygenating thereservoir in which washings are recovered and the possibility ofcontrolled moisture levels through appropriate probing means. It is alsoimportant to mention that the design of the second embodiment allows oneto partially operate the apparatus and to replace decontaminated soil byother contaminated masses in a relatively easy fashion.

The process of the present invention provides means to allow maximumcontrol of the conditions favoring efficient decontamination ofparticulate solids while eliminating the risk of contaminants migrationto surrounding ground waters. Furthermore, the availability of largeamounts of bacteria through multiple inoculation allows the possibilityto maintain optimal biological activity throughout the process.

The following examples are introduced to illustrate rather than limitthe process of the present invention.

EXAMPLE 1

Biological remediation of shoreline oily waste from a marine spill.

On May 8, 1988, the collision of the oil tanker Czantoria with a docklocated in the port of Quebec city resulted in a 2000 to 3000 barrelsspill of light crude oil in the St-Lawrence river. Subsequently, twotypes of shorelines were affected. The first type of shoreline consistedof beaches mostly composed with granular materials such as sand, rockand silt. The second type of shoreline consisted of marshes containingvarious types of weeds.

Oily waste was collected separately from those two types of shorelinesfor subsequent treatment and disposal. Approximately 300 m³ of materialwas contaminated with up to 30% of oil and could not be disposed of inlandfill sites or local incinerators.

The decontamination operation was done in two phases using the systemillustrated in FIG. 1; a first phase of 10 days in the fall of 1988, anda second phase from May to November 1989. During the first phase, twopiles of waste were placed on the impervious surface; one pile ofgranular material (sand, soil, rocks and silt), and one pile of weedsand woody material. The site was operated for 10 days from Nov. 29, 1988to Dec. 9, 1988. The operation was ended because it was no longerpossible to prevent water freeze-up.

The site was re-opened in May 1989. In June, it became evident thatwater flow in the granular pile was no longer suitable for an efficientactivity. The site was then redesigned to contain only one mixed pile.Mixing was done again in July and in September based on temperaturevariations analysis.

In both phases, a culture medium containing phosphorous and nitrogensources, a biodegradable surfactant having an alcohol polar substituantto increase the mobility of the contaminants, at least one hydrocarbondegradating the bacterial strain at a concentration of 10⁶ and asuitable co-substrate at a concentration level of 100 ppm was used.

The pH of the contaminated soil was corrected to be within the range of6 to 8 by treating the soil with dolomitic lime. Inoculation wasconducted every day during which approximately 240 liters of the mediumwere sprayed at a rate of 20 l/min for 2 hours.

The site was closed on November 1989 when contamination level was below1%.

Evaluation of the various parameters of the process 1. Temperature

It has often been said that bioremediation only works within a verynarrow time window in cold latitudes because of problems in maintainingsuitable temperature. This is likely to be the case for landfarming whena thin layer of waste is spread on a wide surface. However, the designof the present invention works at much colder temperatures than what isconsidered to be the limit for landfarming. In fact, it seems that theprocess is efficient even at -20° C. The reason for this increasedefficiency is the fact that compressed air is warmer and the pilethickness generates friction that increases air temperature furthermore.Also, heat created by biological activity is less susceptible todissipation in ambient air. FIG. 2 illustrates this phenomenon for thefirst 10 days of operation. Sharp decreases in the pile temperature arecaused by periodical addition of nutrient solution.

FIG. 3 shows the temperature curves for the remainder of the operation.It is important to note that the pile temperature increases sharplyright after each mixing operation. The decrease in September was causedby operating the compressor on suction mode.

FIG. 4 illustrates the temperature variations during 13 days while thecompressor was operated in this fashion. Even though lower than duringthe compressed air mode, the pile temperature was always kept aboveambient temperature. One of the most important features of thisbioremediation process is the stability of the pile temperature when indiurnal cycles.

Oil and grease degradation

FIG. 5 shows the evolution of oil and grease concentrations. The first10 days represent operation in the fall 1988 and the remaining 160 daysbeing related to the 1989 operation. Up to the 68th day, two curves areshown, one for the granular material and one for weeds. From the 69thday, only one day is shown representing the mixed pile, oil and greaseconcentration.

Initial concentrations were respectively 30.5 and 3.9% in weeds andgranular material. Before mixing the two piles, concentrations were downto 10.0 and 1.7%. Average concentration of the mixed pile was 6.3%,reduced to 1.1% at the end of the operations.

It must be pointed out however that oil and grease analysis is onlyperformed on fine materials. Table I shows the breakdown of materialsfound in the combined waste pile. This was obtained by analyzing arepresentative sample of 20 kilograms. Rocks and large pieces of woodymaterial were not tested for oil and grease. Since these can beconsidered very slightly contaminated--oil and grease % weight close to0--their relative contribution to the pile (50%) allows a redistributionof the tested level of 1.1% of the entire waste, bringing it closer to0.52%.

Leachate test results indicated that the remaining material would notcontaminate soil and groundwater. All tests showed ≦0.2 ppm oil andgrease; acceptable limits for sanitary landfill sites is 15 mg/kg.

                  TABLE 1                                                         ______________________________________                                        Fine materials (weeds, clay, sand)                                                                  47.3%                                                   Large rocks (>5 cm diam.)                                                                           15.4%                                                   Small rocks (<5 cm diam.)                                                                           31.4%                                                   Coarse woody material  5.9%                                                   ______________________________________                                    

Polyaromatic hydrocarbons (PAH's)

FIG. 6 and Table 2 show the progression of PAH's degradation during theprocess. The chromatograms clearly show the disappearance of thecharacteristic PAH absorption peaks. No single PAH was found to be inexcess of the regulated level for residential areas.

The low value for initial naphthalene concentration (3.8 ppm) is anindication that much of the original volatile PAH's were lost to theatmosphere during the first days of the spill.

                  TABLE 2                                                         ______________________________________                                        PAH's degradation                                                             P.A.H.          NOV. 88  JUNE 89    NOV. 89                                   ______________________________________                                        Naphthalene      3.8     n.d.       n.d.                                      Acenaphtylene   25.6     0.6        n.d.                                      Acenaphthene    20.7     3.4        0.4                                       Fluorene        45.8     3.1        1.4                                       Phenanthrene    97.6     14.5       1.8                                       Anthracene      27.7     --         0.8                                       Fluoranthene     6.5     0.6        1.2                                       Pyrene          34.2     18.0       1.0                                       Benzo(a)fluoranthene                                                                          15.5      .06       0.4                                       Benzo(b)fluoranthene                                                                           5.3     0.7        0.1                                       Benzo(k)fluoranthene                                                                          --       1.4        1.0                                       Benzo(a)pyrene   0.5     0.7        0.3                                       TOTAL PAH       283.2    44.0       8.5                                       ______________________________________                                    

Biological activity

At various times during the operation biological activity was measuredthrough bacteria count. At all times count was maintained above 1×10⁷units/g soil, with a peak count of 3.6×10⁹ units/g in June 1989. Throughgrowth analysis it was found that 100% of the microorganisms present inthe soil had affinity for petroleum hydrocarbon substrates. Hence, ininstances where the microflora in the contaminated soil is abundantbecause of a large variety of carbon sources as it is the case in thisexample, the addition of microorganisms may not be necessary or usefulbecause of competition from strains already present in the mass.

The present example demonstrates that bioremediation can be an efficientmean of degradating and disposing of oily waste from marine spillshoreline cleanups. In fact, disposal costs have been shown to be about4 to 8 times less than transportation and disposal at a landfill site.

Content in oil and grease can be reduced to values under 1% withleaching potential of close to 0 ppm. The actual requirements forsanitary landfill in Quebec are respectively 5% and 15 mg/kg for oil andgrease, and leaching. Hence, these requirements were met when using theprocess of the present invention. It seems that some of the key factorsthat were found to be of considerable importance when operating thebioremediation process of the present invention were the following:

sound management practices and tight control are essential to achieveefficient bioremediation;

good water drainage is essential to maintain low compaction and adequateair circulation;

constant temperature measurement is necessary to accurately monitorbiological activity;

periodical mixing is indicated from temperature readings must beperformed to maintain the system at its peak performance level.

EXAMPLE 2 Biorestoration of soil contaminated with PCPs.

The system used in this example was the system shown in FIGS. 7-9.

The contaminated soil to be treated was siltous sand having a relativelyweak permeability. Extended spraying periods were thus required in orderto allow suitable absorption of the nutrients medium to avoid liquidbuild-up at the surface of the pile. The compaction levels of the pileincreased as its permeability decreased, and hence turning the pile wasessential to maintain a soil structure that was as loose as possible.

120 cubic meters of the contaminated soil which initially exhibited PCPlevels of 115 ppm was placed on an impervious concrete surface having athickness of 50 mm and an inclination of 1%. The air distribution systemlocated between the concrete surface and the contaminated pile wascovered with gravel. This gravel bed was useful to increase airdiffusion and the drainage of the pile. It turned out that it was onlymoderately effected when the pile was turned. A fabric cover made offabrene was deposited on the contaminated mass. It turned out to besuitable to control rain water and to help allow efficient recirculationof air. Also, the fabric covers increased the heating efficiency of thepile.

The concrete surface was in fluid communication with a 2000 literunderground recovery reservoir. A culture medium containing phosphorousand nitrogen sources, a biodegradable surfactant having an alcohol polarsubstituent to increase the mobility of the contaminants, a PCPdegradating bacterial strain at a concentration of 10⁶ and a suitableco-substrate at a concentration level of 100 ppm was used.

The pH of the contaminated soil was corrected to be within the range of6 to 8 by treating the soil with dolomitic lime in an amount of 500 kgfor 120 m³ of soil prior to the first irrigation inoculation. The firstinoculation was conducted on Jul. 13, 1990 during which approximately240 liters of the medium were sprayed at a rate of 20 l/min for 2 hours.Subsequently, a soil sample was analyzed previous to each inoculation.On September 25th to October 12th, the contaminated mass was irrigatedevery day for 2 hours at a rate of 20 l/m and 3 times in the week ofOct. 14th, 1990 at a rate of 20 l/m for 2 hours. The last spraying wasdone on Friday, Oct. 19th, 1990 at a rate of 20 l/m for 2 hours. Thespraying rate was augmented to increase the concentration ofmicroorganisms in the contaminated pile toward the final stages of thedecontamination process to enhance degradation probabilities as thecontaminant concentration had been reduced to relatively low levels.

The pile was turned every 2 to 3 weeks, depending on the compactionstate of the pile and the availability of the required machinery.

From Sep. 14th, 1990, a heating element was introduced to maintain thetemperature of the contaminated pile at a suitable level. The heatingwas controlled by a thermostat so that the air provided to thecontaminated soil did not exceed 25° C. Heating was required until theend of the decontamination process on Nov. 15th, 1990.

Certain sediments have a tendency to end-up in the recovery reservoirbut they are recirculated after spraying. In fact, after spraying,allowing the pump to be constantly operated in the reservoir,homogenizes the medium. Finally, it is to be noted that during the hotsummer months, operation losses were compensated by filling thereservoir with fresh water. The results obtained are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                                        DAYS    PCP   O & G   WASHINGS                                #    DATES      Bite    (ppm) (ppm)   PCP (ppm)                               ______________________________________                                        1    900707      0      115   700     n/d                                     2    900714      7      51,4  600     n/d                                     3    900718     11      36,4  400     n/d                                     4    900727     20      8,0   n/d     7,6                                     5    900810     34      7,4   n/d     n/d                                     6    900817     41      13,8  300     n/d                                     7    900829     53      10,8  n/d     n/d                                     8    900907     62      13,6  n/d     n/d                                     9    900914     69      12,7  n/d     0,6                                     10   900925     80      6,6   n/d     n/d                                     11   901001     86      6,5   n/d      0,25                                   12   901009     94      4,5   n/d     n/d                                     13   901015     100     4,6   n/d      0,02                                   14   901026     111     3,5   n/d     n/d                                     15   901115     131      1,9* n/d     n/d                                     ______________________________________                                         *NOVALAB Analysis, Montreal.                                                  Note 1:                                                                       In average, the soil fraction is equal or lower than 2,38 mm (fraction on     which the analysis were conducted) represented in weight 35% of the           samples. 2/3 of the soil is thus constituted by large particles.              Note 2:                                                                       The analysis methods are the following:                                       For PCP: EPA 8040                                                             For oil and greases: Standard Methods 503, A & E.                             n/d: nondetectable.                                                      

FIGS. 10-12 respectively show the reduction in PCP concentration in thecontaminated pile, the reduction in oil and greases and the reduction inPCP concentration in the washings. It can be seen from the table thatthe inoculations made between the beginning of August and mid-Septemberhad a relatively low efficiency as the contamination level of about10-12 ppm appeared to be maintained. The use of a co-substrate in thecontaminated mass helped to provide optimal bacterial activity and thisallowed further decrease in PCP level.

Concentration levels of oils and grease in the contaminated mass alsodecreased with time. Since a concentration of 300 mg/kg is well underresidential criteria for the province of Quebec, measurement of thisparameter was interrupted at this level. The few measurements provideddemonstrate the possibility of obtaining relatively efficientelimination of oil and grease contaminants in soil through biotreatment.

Analysis of the washings from the treated pile showed a decrease in thepresence of PCPs as the decontamination process proceeded to completion.The last analysis performed indicated a PCP level of 0.02 mg/l in thewashing. This concentration usually meets standard waste levelsrecommended by Canadian cities for phenol. Oxygenation of the reservoirprovided means to further degrade residual PCPs in the washings.

Hence, the final analyses indicated a 1.9 ppm residual concentration inPCP, which is substantially under the 5 ppm norm indicated forindustrial lands in the province of Quebec. All the analyses wereperformed on soil particles having 2.3 mm or less in size. The resultsobtained are therefore overestimated as soil particles of this sizerepresent only 1/3 of the total weight of the contaminated mass whilepractically containing all the PCP contaminants. Whereas the smallerparticles retain contaminants through absorption and adsorption, largerparticles are only slightly affected because of a surface/volume ratiothat is much smaller. Also, large particles are relatively easy to treatthrough repeated washings. In fact, if one considers the soil to betreated as a single entity, it would seem realistic to conclude that itsactual decontamination level could be closer to three times lower thanthe levels indicated in Table 3.

A reduction in PCP concentration in contaminated soil from 115 ppm to1.9 ppm was achieved. This represents a contaminant reduction rate of98% over a four month period. This is much higher than what has beenpreviously reported in the prior art and would appear to open up newpossibilities for efficient biological degradation of highly undesirablecontaminants.

Claims to the invention follow.

I claim:
 1. A method for the biodegradation of organic contaminants in amass of particulate solids, said method comprising:a) providing acontaminated mass of particulate solids on an impervious surface influid communication with an impervious recovery reservoir, saidimpervious surface having operating thereon an air supply means toprovide suitable and continuous oxygenation of said mass; b)periodically irrigating said mass by applying on its surface a culturemedium comprising at least one bacterial strain and co-substratesthereof, said bacterial strain having the ability to degrade saidorganic contaminants, said medium being drained through said mass andwashings recovered in said reservoir to be reused to irrigate said mass;and c) periodically mixing said mass and controlling the temperature andmoisture level of said mass to monitor the biological activity of saidbacterial strain and to maintain said activity at levels suitable forsaid bacterial strain to degrade said contaminants in said mass.
 2. Amethod according to claim 1, wherein said impervious surface is sloped.3. A method according to claim 1, wherein said air supply meanscomprises a series of perforated conduits, said conduits being connectedto an air compressor operable in suction mode.
 4. A method according toclaim 3, wherein said series of perforated conduits are located inside agravel bed.
 5. A process according to claim 3, wherein said compressorcomprises an activated charcoal filter system operated when saidcompressor is in the suction mode.
 6. A method according to claim 3,wherein said air compressor also is operable in compressed mode.
 7. Amethod according to claim 6, wherein said series of perforated conduitsare located inside a gravel bed.
 8. A method according to claim 6,wherein said compressor comprises an activated charcoal filter systemoperated when said compressor is in the suction mode.
 9. A processaccording to claim 1, wherein said culture medium further comprisesnutrients and surfactants.
 10. A process according to claim 1, whereinsaid culture medium is provided from a fermentation unit allowing tomaintain continuous growth of said bacterial strain or from saidimpervious recovery reservoir.
 11. A method according to claim 10,wherein said culture medium applied on said mass is pumped from saidfermentation unit through a plurality of sprinklers or through aplurality of perforated double-partition irrigation tubes in fluidcommunication with said fermentation unit or said impervious recoveryreservoir, said sprinklers or tubes being located above said mass.
 12. Amethod according to claim 11, wherein said culture medium irrigatedthrough said mass is reused by being pumped from said reservoir throughsaid sprinklers or tubes.
 13. A method according to claim 11, whereinthe impervious surface further has operating thereon an air suctionmeans to provide suitable and continuous oxygenation of said mass and toremove undesirable vapor emissions from said mass.
 14. A methodaccording to claim 1, wherein the temperature and moisture level of saidmass are measured by a plurality of thermocouples and probes inserted insaid mass.
 15. A method according to claim 1, wherein said mass iscontaminated with hydrocarbons.
 16. A method according to claim 1,wherein said mass is contaminated with PCBs.
 17. A method according toclaim 1, wherein said reservoir is oxygenated to maintain bacteria insaid washings.
 18. A method according to claim 13, wherein saidoxygenation is realized by either blowing air in said reservoir or byadding hydrogen peroxide to said medium.
 19. A method according to claim1, wherein said air supply means further removes undesirable vaporemissions from said mass.